This invention relates generally to drying of paper-related web or sheet form materials; and more particularly, to process and apparatus for high-intensity drying of fiber webs, such as newsprint, printing and writing papers, packaging paper and paperboard, or linerboard, as they are formed on a paper machine.
Paper drying represents one of the most energy-intensive operations in the paper mill. The objective of the drying section is to remove residual moisture in the pressed sheet at an efficient production rate with minimum usage of steam. For most dryers in use today, this is accomplished by contacting the paper web against a series of steam-heated, rotating cylinders or drums which are contained inside a machine room, that is ventilated by ambient air, to remove sheet moisture. Heat for evaporating moisture comes from steam, which condenses on the inner surface of the dryer cylinders. Condensate formed inside the cylinders must be removed by complex siphons that are often prone to thermal losses due to steam blow-by.
The driving force for heat transfer to the paper web is related to the difference between the cylinder steam temperature and the web evaporation temperature, and is limited by several thermal resistances, which include the condensate layer formed on the inside surface of the cylinder, scale, which forms on both the internal and external surfaces of the cylinder, and an air layer, which forms between the cylinder surface and the paper web. Because of these thermal resistances, the dryer heat flux rate is typically limited to values of 3-5 pounds of steam evaporated per hour per square foot of total cylinder area (3-5 lb/ft.sup.2 /h) in the prior industrial art. These low heat flux rates result in the need for many expensive drying cylinders, which occupy a large space within the machine room.
Since the paper web temperature is generally kept below the ambient boiling point, vapor removal from the web is mass-transfer driven by the vapor partial pressure gradients between the sheet surface and the ventilation air. The paper web picks up sensible heat while in contact with the cylinder, and flashes off steam in the open draw between the top and bottom cylinders, causing the sheet to spontaneously cool, and therefore become ready to pick up sensible heat again.
To maintain sufficient partial pressure gradients, air humidity in the dryer spaces is controlled by passing large quantities of heated ventilation air through the machine room. This involves the use of expensive air handling equipment, such as fans, ducts, machine room hoods, and air heat exchangers. Because the ambient air is heated to the evaporation temperature, a 10%-25% loss in dryer thermal efficiency occurs and results in dryer steam consumption of typically over 1.5 pound steam per pound of water evaporated.
In the prior art, the paper web passes in a serpentine fashion over an array of rotating cylinder dryers, and a restraining felt is used to enhance contact between the paper web and the heated cylinder surfaces. Despite the use of restraining felts, the paper web contacts only a portion of the available cylinder dryer circumferential area. Because of the use of restraining felts, pockets are formed between the restraining felt and paper web which must be evenly ventilated to maximize evaporative drying rates and to minimize cross-directional (CD) property variations that otherwise result in operational and quality problems related to reel building, calendering, and converting. Restraining felts are prone to plugging by paper fibers and may require on-line cleaning. They also experience wear during the course of normal operation, which necessitates maintenance and periodic replacement of the felt materials.
The development of new drying apparatus for increasing drying rate has been an industry objective for several decades. It is now understood that higher drying rates can be achieved by subjecting the paper sheet to sufficiently intensive and prolonged heat input so as to boil moisture from the web. This method is known as high-intensity drying.
Several attempts have been made to achieve higher heat intensities for drying paper by increasing the temperature of the heat transfer surface, increasing the contact time of the web against the heat transfer surface, and/or increasing the applied compressive force of the web against the heat transfer surface. All of these attempts have exclusively relied on the use of a solid surface, such as a belt or press roll, to transfer heat to the paper web.
For instance, the "PIRA" press drying unit ("PIRA" 1987) was developed to simulate a long nip press operating at high temperature and at speeds up to 400 m/min. The machine relies on a tensioned steel belt to provide a maximum nip pressure of about 400 kPa over a length of 0.3 m. A press roll located underneath the belt can apply additional pressure over a short nip length (2.5 mm). A multiple-nip system (Garcia 1987) has been operated at speeds up to 400 m/min, and optimum nip pressures of between 200-300 pli. These systems provide means contacting the paper web against a heat transfer surface under high compressive forces and short contact time, but do not allow drying of the paper product to final reel specifications.
The Tampella "Condebelt" press (Lehtinen 1988) consists of two steel belts--one heated to 150.degree. C. and the other cooled to less than 100.degree. C. The wet web is placed against the hot belt, and water evaporates from the sheet and condenses on the cooler belt. Steam is confined behind the hot belt to supply heat to the paper web and to provide compressive force on the belt and the underlying paper web surface. The "Condebelt" press is capable of achieving increased contact times between the paper web and the heat transfer surface compared to nip press apparatus; but it suffers from engineering limitations, such as the availability of practical methods to seal the steam heat transfer fluid against a large and rapidly moving heat-transfer belt.
Each of these systems are limted by mechanical difficulties encountered when attempting to contact a high-temperature solid surface against a fast moving paper web; and the mechanical systems described above have not yet demonstrated the objective of achieving high-intensity drying at commercial machine speeds for the purpose of producing a dried paper product having the requisite final reel moisture specifications and mechanical qualities using a single drying apparatus of compact dimension, high thermal efficiency and low cost.
Nip press apparatus, which apply high-compressive forces to accomplish high-intensity impulse drying, appear to be limited by sheet delamination and loss of z-directional strength, which has been observed upon release of nip pressure. Nip presses provide inherently short contact drying times, typically less than 0.1 seconds, and are only applicable to the partial dewatering of the paper web, and thus must be coupled with other drying techniques to achieve final reel moisture specifications. Methods described in the literature involving use of heated and cooled belts with mechanical seals to confine the heating and cooling media on opposing sides of the belts do not appear to be commercially practical, due to problems associated with seal tolerances, wear and leakage.
There is need for process and apparatus that overcomes problems and limitations, as discussed above.